In digital images, colors, including gray scale and black and white, are represented at various bit depths. For various reasons, the bit depth may be reduced. For example, film is often digitized during post-production, and the film is often digitized and processed at relatively high bit depths. Relatively high bit depths permit more colors to be represented. In some implementations, the digitized version of a film image has a relatively high bit depth of 10 bits per component (bpc) or higher. Content that is generated as digital images is often rendered at even higher bit depths, such as 16 bpc. One reason for reducing the bit depth is that images having a bit depth of 8 bpc are more desirable for compression, for use on standard definition and high definition consumer DVD players. The reduction of bit depth may be referred to as color quantization.
In areas of a high bit depth image with smooth color gradients, color quantization may produce “bands,” each of which is constant in color, with a small color difference between adjacent bands. Boundaries between such bands may be visible as false contours, also referred to as “banding artifacts”. Methods such as error diffusion aim at reducing the occurrence of false contours during the bit depth reduction process. However, one may be left with a quantized image with visible false contours. In some cases, the banding artifacts are already present in the higher bit depth image.